²⁷Al MAS NMR SPECTROSCOPY STUDY OF Eu²⁺-DOPED AND Dy^3+-CO-DOPED SrAl₄O₇

Received 6 December 2019; accepted 16 December 2019

1. Introduction

Strontium aluminates, doped with rare-earth metal ions, have been studied for a long time for their excellent properties, such as a high quantum efficiency and a long persistence of phosphorescence, as well as due to potential industrial applications in fluorescent lamps, plasma displays, light emitting diodes, etc.  [1–3]. Beside a  very high quantum efficiency and a long persistence luminescence, strontium aluminates doped with Eu²⁺ have a better stability than other alkaline earth aluminates  [4]. Several different strontium aluminate phases are well known, e.g. SrAl₂O₄, Sr₄Al₁₄O₂₅, SrAl₁₂O₁₉ and Sr₃Al₂O₆ [5]. However, a particular one – SrAl₄O₇ – was much less investigated, probably due to the difficulties arising at the preparation of monophasic samples. The amount of phase impurities that can be therein sometimes too small is to be detected by the most widely used methods of characterization (XRD, FTIR, etc.).

Very recently, it has been found that Eu²⁺-doped strontium aluminate SrAl₄O₇ samples exhibit blue-green persistent luminescence at 490  nm while the  co-doping with Dy³⁺ shifts the  maximum of emission to 475 nm and, consequently, the colour towards the blue spectral range [6]. It is well known that the emission of Eu²⁺ is very strongly dependent on the host lattice and can occur from the ultraviolet to the red region of the electromagnetic spectrum [4, 7, 8]. The  purposes of the  present work were to examine the phase purity and to elucidate the possibility and the extent of structural changes in SrAl₄O₇ environment upon doping and co-doping with Eu²⁺ and Dy³⁺ ions using solid-state nuclear magnetic resonance (NMR), namely ²⁷Al MAS (magic-angle spinning) NMR spectra. These complementary data can be useful in trying to explain the blue shift observed in the  SrAl₄O₇:Eu²⁺ emission spectra upon co-doping with Dy³⁺.

2. Experiment

The undoped, 3% Eu-doped and 6% Dy-co-doped SrAl₄O₇ samples were prepared by the solid state-reaction method. The phase purity of synthesized specimens was examined by powder X-ray diffraction measurements. The details of chemical preparation, synthesis and characterization are presented in Ref. [6].

The solid-state ²⁷Al MAS NMR experiments were performed using a 400 MHz Bruker AVANCE III HD spectrometer with a 4 mm-wide bore double resonance HX  CP-MAS probe. The  experiments were performed in 9.4  T magnetic field using an Ascend wide bore superconducting magnet. The Larmor frequency for ²⁷Al was 104.3 MHz and chemical shifts were referenced to 1 M Al(NO₃)₃. The MAS frequency (ν_MAS) was 12 kHz. ²⁷Al MAS spectra were measured applying the single pulse-acquire sequence using the  short duration (2  μs) excitation pulse, the number of scans was 512 and the recycle delay was 10 s.

In addition, the ²⁷Al MAS spectra were acquired with the  rotor synchronized Hahn-echo sequence (π/6 – τ – π/3 – τ – acq) [9], setting τ = 10/ν_MAS (Fig. 1).

The NMR spectra were processed using the Topspin 3.2 software. Some additional processing was carried out using the Microcal Origin 9 package.

3. Results and discussion

The structure of SrAl₄O₇ has been studied in several works [10–12]. The structure was deduced to be a  monoclinic structure with the  space group C12/c1 [10, 11]. The SrAl₄O₇ lattice consists of corner sharing AlO₄ tetrahedral layers and strontium ions, which are embedded in between the layers [2, 13]. Layers of highly interlinked AlO₄ tetrahedra extend in the xy plane and give rise to the enhanced growth speed in the x and y direction. In between the layers, one finds the Sr ions as well as the AlO₆ octahedra that act as bridges [13]. Each Sr atom in SrAl₄O₇ is surrounded by ten O atoms with interatomic distances from 2.50 to 2.79 Å [2]. This compound also has a high-pressure form β-SrAl₄O₇. The crystal lattice of this form consists of a 3-dimensional network of Al(1)O₆ octahedra, and A1(2)O₄ and A1(3)O₄ tetrahedra. The A1–O bond lengths were found to be 1.795 to 1.968 Å for the octahedra and 1.449 to 1.537  Å for the  tetrahedra  [6]. These distances in A1O₄ units are considerably shorter than those in other aluminates.

In literature there are known several synthesis routes used for the preparation of SrAl₄O₇. They are reviewed in Ref.  6. However, the  route of conventional solid-state reactions is seldom reported. Moreover, in some works it was claimed that SrAl₄O₇ cannot be synthesized by the solid-state reaction. In our earlier paper  [6], a  successful synthesis of SrAl₄O₇ samples doped with Eu²⁺ and co-doped with Dy³⁺ by the conventional solid-state reaction method was carried out.

A broad variety of solid-state NMR techniques, such as ²⁷Al MAS, multiple quantum magic-angle spinning (MQ-MAS) and heteronuclear multiple quantum coherence (HMQC) spectroscopy, have been effectively applied elucidating very fine structural details in series of ions-doped aluminates [1, 14–19]. Therefore, in the present work the ²⁷Al MAS NMR was applied in order to complementarily characterize the synthesized SrAl₄O₇ using the conventional solid-state reaction method as well as 3% Eu-doped and 6% Dy-co-doped SrAl₄O₇.

As the major factor that controls the ²⁷Al chemical shifts δ(²⁷Al) is the atomic structural environments of aluminum atoms, i.e. the nearest-neighbour coordination geometry, the  ²⁷Al MAS NMR spectra provide useful, sometimes even unique information on Al sites and coordination numbers. However, not always all of the ²⁷Al resonances corresponding to different Al sites are well resolved in the  spectra. Going through literature data  [4, 18, 19 and Refs. cited therein], it can be stated that in most Al–O environments, the ²⁷Al NMR peaks of 6-coordinated Al (AlO₆) appear in the range covering –20 to +15 ppm and are well separated from the signals from the 4-coordinated Al (AlO₄) that appear at 50 to 145 ppm. It looks that the interchange of ions Sm ↔ Cs ↔ Sr in various aluminates does not have too much influence on this sequence and the range of variation of ²⁷Al chemical shifts.

The ²⁷Al MAS NMR spectra of pristine as well as 3% Eu-doped and 6% Dy-co-doped SrAl₄O₇ samples studied in the present work are shown in Fig. 2. The Hahn-echo pulse sequence is typically used for solid-state NMR measurements when the  single-pulse spectrum experiences a  strong background signal due to, e.g. long probe dead time or ringing effects. By changing echo time it could also be used as a relaxation filter, in other words, fast relaxing spectral components could be filtered out if two or more coexisting species, which relax at different rates, are present in the sample. Note that a higher resolution and reduction of spinning sideband intensities were achieved applying the Hahn-echo pulse sequence in the present case (Fig. 2(a)). The peaks of 4- and 6-coordinated Al are well resolved and easily recognized in the spectra. No significant changes in the ²⁷Al chemical shifts upon Eu²⁺ and Dy³⁺ doping are seen. Only the relative intensities can be noted, and they are more pronounced for 4-coordinated Al (Fig. 2(b)). A very asymmetric shape of AlO₄ signals with the  tails to lower values of ²⁷Al chemical shifts point towards the  presence of a local disorder and thus a broad distribution of quadrupolar coupling constants and chemical shifts.

It is known that the  5-coordinated Al (AlO₅) yields δ(²⁷Al) in the range +15 to +35 ppm. This was observed in the ²⁷Al NMR spectrum of glassy YAG–4Si, however not seen in the  pure crystalline YAG and in the  spectra of annealed YAG– 4Si  [18]. It means that this spectral feature can be used evaluating the  structural disorder and phase purity in the  sample. However, note that a certain polemic around the existence and detection of the ²⁷Al NMR signals from AlO₅ sites run in the literature [16, 17, 19]. The AlO₅ signal, if it is at all present in the aluminate under investigation, is often obscured by the overlap with strong AlO₆ signals. The most powerful technique to reveal the presence of AlO₅ is MQ-MAS. The study applying this technique on the synthesized materials is in progress.

4. Concluding remarks

1. The  ²⁷Al MAS NMR experiments have shown that the Eu²⁺ with Dy³⁺ ion doping carried out using the solid state-reaction method did not affect the bulk structure and the local Al environment in SrAl₄O₇.

2. The  spectral features observed in the  range between 4- and 6-coordinated Al peaks (viz. 20– 40  ppm) demand one to recognize that certain phase imperfections are present, most probably the  amorphous/glassy domains appear. Also note that the chosen solid state-reaction regime does not alter it, i.e. the spectral features that supposedly may reflect the phase purity were the same in all studied samples.

3. The ²⁷Al MAS NMR technique appeared to be a  very effective tool for the  analysis of phase perfectness. Such amount of phase impurities was too small to be detected by widely used XRD or FTIR methods [6].

4. Large shifts of the emission maximum (from 490 to 475 nm) by co-doping of SrAl₄O₇:Eu²⁺ with Dy³⁺ cannot be caused by the  local environment changes upon the doping. Since the main factor governing the emission of Eu²⁺ is the crystal field surrounding the ion, one possible explanation might be that the  charge disbalance induced by 3⁺ ions influences the strength of the crystal field around Eu²⁺ (without noticeably affecting the structure).

Acknowledgements

This project was supported by the European Social Fund (Project No.  09.3.3-LMT-K-712-02-0114) under Grant Agreement with the  Research Council of Lithuania. The  authors acknowledge the  Center of Spectroscopic Characterization of Materials and Electronic/Molecular Processes (SPECTROVERSUM, www.spectroversum.ff.vu.lt) at the  Lithuanian National Center for Physical Sciences and Technology for the  use of NMR equipment. The authors thank Prof. Aivaras Kareiva for helpful discussions.

References

Eu²⁺ LEGIRUOTŲ IR Dy³⁺ KOLEGIRUOTŲ SrAl₄O₇ TYRIMAI ²⁷Al MAS BMR SPEKTROMETRIJOS METODU

Santrauka

Eu²⁺ legiruoti stroncio aliuminatai SrAl₄O₇ pasižymi mėlynai žalia išliekančiąja liuminescencija ties 490 nm, o juos kolegiravus Dy³⁺ jonais emisijos maksimumas pasislenka link 475  nm. Kietafazių reakcijų metodu buvo paruošti nelegiruoti, 3 % Eu legiruoti ir 6 % Dy kolegiruoti SrAl₄O₇ mėginiai, kurie ištirti taikant kietojo kūno ²⁷Al MAS („magiško kampo sukimo“) BMR spektrometrijos metodą. Naudotos pavienio impulso ir Hahno sukinių aido impulsų sekos. Nustatyta, kad SrAl₄O₇ legiravimas Eu²⁺ ir Dy³⁺ jonais neturi įtakos kristalinei struktūrai visame bandinio tūryje bei lokaliai Al atomų aplinkai. Tai reiškia, kad toks pastebimas emisijos maksimumo poslinkis SrAl₄O₇:Eu²⁺ kolegiruojant Dy³⁺ nėra nulemtas lokalios kristalinės struktūros pokyčių. Kita vertus, BMR spektro dalis, stebima tarp keturių ir šešių koordinuotų Al signalų (20–40 m.d.), parodo, kad šiuose bandiniuose yra kitų fazių priemaišų, ir, labiausiai tikėtina, amorfinių / stikliškų domenų. Pažymėtina, kad tokie maži fazinių priemaišų kiekiai nebuvo aptikti įprastais ir bene dažniausiai šiam tikslui pasiekti taikomais rentgeno difrakcijos (XRD) bei Furjė vaizdavimo infraraudonosios spektrometrijos (FTIR) metodais. Tai rodo, kad ²⁷Al MAS BMR metodas gali būti labai efektyvus įrankis kontroliuojant stroncio aliuminatų serijos junginių fazinį grynumą.